The cost of fuel cells must be reduced dramatically to become commercially viable on a larger scale. The cost of the flow field plates, including the cost of forming the flow field onto the plate, represents a significant portion of the total cost within a fuel cell. Therefore, cost reduction of the flow field plate is imperative to enable fuel cells to become commercially viable on a larger scale. The cost reduction can be manifested in several ways including reducing the cost of the materials that are used to make the plate, reducing the manufacturing cost associated with making the plate, and/or improving the function/performance of the plate within a fuel cell so that the same fuel cell can produce electrical power more efficiently and/or produce more electrical power within the same fuel cell.
A typical Polymer-Electrolyte-Membrane (PEM) fuel cell comprises several components. These components typically include a membrane, catalyst layers on the anode and cathode sides of the membrane known as the gas diffusion electrodes, and gas diffusion backings on each side. The membrane, electrode layers and gas diffusion backings are laminated together to create the membrane electrode assembly (MEA). Each MEA is sealed between two thermally and electrically conducting flow field plates. Each cell is then “stacked” with other cells to achieve the required voltage and power output to form a fuel cell stack. Each stack is subjected to a compressive load to ensure good electrical contact between individual cells.
In operation, fuel is introduced on the anode side of the cell through flow field channels in the conductive flow field plates. The channels uniformly distribute fuel across the active area of the cell. The fuel then passes through the gas diffusion backing of the anode and travels to the anode catalyst layer. Air or oxygen is introduced on the cathode side of the cell, which travels through the gas diffusion backing of the cathode to the cathode catalyst layer. Both catalyst layers are porous structures that contain precious metal catalysts, carbon particles, ion-conducting NAFION® particles, and, in some cases, specially engineered hydrophobic and hydrophilic regions. At the anode side, the fuel is electrochemically oxidized to produce protons and electrons. The protons must travel from anode side, across the ion-conducting electrolyte membrane, finally to the cathode side in order to react with the oxygen at the cathode catalyst sites. The electrons produced at the anode side must be conducted through the electrically conducting porous gas diffusion backing to the conducting flow field plates. As soon as the flow field plate at the anode is connected with the flow field plate at the cathode via an external circuit, the electrons will flow from the anode through the circuit to the cathode. The oxygen at the cathode side will combine protons and electrons to form water as the by-product of the electrochemical reaction. The by-products must be continually removed via the flow field plate at the cathode side in order to sustain efficient operation of the cell. Water is the only by-product if hydrogen is used as the fuel while water and carbon dioxide are the by-products if methanol is used as the fuel.
Conductive flow field plates comprise the outer layers of a fuel cell and serve a number of functions: they provide structural integrity to the fuel cell; protect the fuel cell from corrosive degradation over the operating life of the fuel cell; and, most importantly conduct electrons and heat from the interior of the fuel cell to the exterior. Conductivity at the interface between the flow field plate and the outermost interior layer, i.e., gas diffusion layer, is critical for minimizing resistance in the fuel cell.
Because of the unique set of performance requirements of conductive flow field plates and the aggressive conditions inside the fuel cell, the material options for constructing conductive flow field plates are limited. In general, graphite has been used for conductive flow field plates because of its high electrical conductivity and resistance to corrosion. Graphite however is typically produced in 6 mm thick slabs, adding both weight and bulk to the fuel cell and decreasing its power density when in use.
Carbon/graphite fillers in plastic polymers have been identified as a promising alternative to graphite in manufacturing conductive flow field plates. Processes for preparing such plates are disclosed in U.S. Pat. No. 4,124,747 to Murer and Amadei, U.S. Pat. No. 4,169,816 to Tsien and U.S. Pat. No. 4,686,072 to Fukuda.
While these carbon/graphite filler plates provide increased durability and flexibility to the fuel cell, the composition of carbon/graphite filler plates provides less than superior conductivity and resistivity (both bulk resistivity and through plane resistivity) properties. Attempts have been made to reduce the resistivity of a molded plate by machining the surface of the molded plate to eliminate the polymer rich skin layer from the surface of the plate. Such machining processes however are time consuming and expensive.
Conductive fuel cell collector plates have been made with different kinds of blends, including the following blends:
a. Graphite filled liquid crystal polymer plates;
b. Graphite filled polyvinylidene fluoride (PVDF) plates; and
c. Graphite filled thermoset (vinyl ester) plates.
The use of graphite filled binary polymer blends for improving conductive properties alongside other mechanical and thermal properties is well known. Some of the work done in this area includes:
a. Wu et al 2001 (Wu G, Miura T, Asai S, Sumita M, “Carbon Black-Loading Induced Phase Fluctuations In PVDF/PMMA Miscible Blends: Dynamic Percolation Measurements”, Polymer 42 (2001) 3271-3279) investigated carbon filled polyvinylidene fluoride / poly(methyl methacrylate) (PVDF/PMMA) blends. They found that the carbon black induces phase fluctuations in PVDF/PMMA blends to reduce percolation threshold.
b. Del Rio et al 1994 (Del Rio C, Acosta J L, Polymer 35 (1994) 3752) found that carbon black and Cu compatibilize polyvinylidene fluoride/polystyrene (PVDF/PS) systems and also result in improvement of electrical properties.
c. Del Rio et al 2000 (Del Rio C, Ojeda M C, Acosta J L, “Carbon Black Effect On The Microstructure Of Incompatible Polymer Blends”, European Polymer Journal 36 (2000) 1687-1695) did a detailed study on the morphology and thermal properties of carbon filled polyvinylidene fluoride / polyamide 6 (PVDF/PA6) blends. They found that carbon black induces partial compatibilization and modifies isothermal crystallization kinetics in these blends.
The disclosures of all patents/applications and documents referenced herein are incorporated herein by reference.
There remains a need for a new composition for the conductive fuel cell collector plates with reduced resistivity and lighter weight without compromising necessary plate conductivity and strength. Also, it is preferred to make the plates with cheaper materials to bring down the cost of the fuel cell.